As printing systems in which images are formed and printed on a printing medium (paper) by jetting liquid dots from an inkjet head, known systems include a serial print system in which a recording head of an inkjet type having a plurality of inkjet nozzles is mounted on a carriage so as to be reciprocal and a paper is fed little by little in a perpendicular direction (sub-scanning direction) for recording while scanning the inkjet head in a widthwise (main-scanning) direction of the paper (of regular size), and a line print system in which inkjet nozzles are arranged in a line on a recording head of an inkjet system so as to correspond to one line of the paper and recording is performed while scanning in a paper feed direction with the recording head kept standstill.
Among them, high-speed printing is possible with printers of the line print system, which are used as an on-demand inkjet line printer. Not to mention with the serial print system, with the inkjet recording system using an inkjet line printer, a dot recording method of a multi-pass system is generally adopted in which thin-out images are recorded with time lags to prevent the image from deteriorating due to uneven density or ink blurring. Various proposals have been made to improve the image quality in forming images.
The deterioration of image quality with an inkjet line printer is caused by unevenness in the volume and direction of ink jetted from the ink nozzles with the nozzles and resulting unevenness in the size and position of dots. Unevenness in the dot position results in poor uniformity of the distance between dots. The smaller the distance, the higher the density whereas the larger the distance, the lower the density. Sometimes a white stripe appears, thus impairing image quality. Also, unevenness in the dot size causes the difference in the density between the adjacent dots, forming stripes and thus deteriorating the image quality.
As a dot recording method for preventing the deterioration of image quality, patent publication 1 (Japanese patent publication 10-138520) discloses a method of dot recording an image with two or four rotations of the rotary drum. With the method of printing with two rotations of the rotary drum, jet nozzles of a number corresponding to the pixel number in the main scanning direction to be printed on the paper mounted on the drum are arranged in the main scanning direction, dot recording is done with the first rotation of the drum on the paper moving with the rotation of the drum from the nozzles on every other line in the main scanning direction, and dot recording is performed with the second rotation of the drum from the nozzles in the remaining lines.
With the method of printing with four rotations of the drum, dot recording is done with the first rotation of the drum every other dot in the main scanning direction and every other dot in the sub-scanning direction, too, with the second rotation in spaces between the dots, with the third rotation in the blank portions in the main scanning direction, and with the fourth rotation in the blank portions in the sub-scanning direction, so that all the pixels are recorded with four rotations of the drum. With such a dot recording method, the ink drying time can be prolonged until dot recording for all the dots is complete. Thus it is suited for high quality printing rather than high speed printing.
The patent publication 2 discloses a dot recording method in which the recording heads are arranged so as to be movable in the main scanning direction and the jet nozzles are arranged at a pitch twice the pixels to be recorded, and dot recording is done with the first rotation of the drum every other dot in the main scanning direction and every other dot in the sub-scanning direction, too, and with the second rotation of the drum dot recording is done with the recording head moved by one dot so that the recording position for the basic resolution of dot will be shifted between the even number lines and the odd number lines in the main scanning direction. This dot recording method is said to be effective to reduce the formation of stripe-like unevenness in density in printing requiring intermediate graduation.
The patent publication 3 discloses a dot recording method by multi-pass printing using a jet nozzle printer of a serial print system. With this dot recording method, a plurality of jet nozzles are arranged in a direction perpendicular to the scanning direction of a carriage which moves in a widthwise direction of the paper. With this dot recording method, various multi-pass multiple speed printings are possible from 2-pass double speed printing to 4-pass quadruple speed printing. With this method, by setting the scan speed for the second recording mode higher than that for the first recording mode, double speed or quadruple speed printing is possible.
The dot recording methods disclosed in Patent publications 1 and 2 are multi-pass dot recording methods in which a drum is rotated and a plurality of jet nozzles are arranged in the main scan direction, but they mainly aim to prevent the deterioration of quality and prevent unevenness in density and ink blurring by dot recording the remaining pixels passed in two or four rotations of the drum with some time delays. They are not designed for dot recording or printing at double or quadruple speed.
The Patent publication 3 discloses a dot recording method of serial print type, multi-pass system which permits high-speed printing such as double or quadruple speed in the main scanning direction. But, with the serial print system, inkjet nozzle heads carried on a carriage are moved for dot recording, and each nozzle head is provided with a plurality of nozzles, but the length of the nozzle head is limited to several fractions of the width of the paper. So a plurality of nozzles are provided, but the number is much less than on a line printer. Therefore, its speed is limited to double or quadruple speed.
If one intends to apply the multi-pass dot recording method for a serial printer to a line printer with a rotary drum, it is not applicable because a serial printer is not a rotary drum type. Also, a single paper is mounted on the rotary drum of the line printer disclosed in the Patent publications 1 and 2. So it is not possible to dot record and print continuously and efficiently on a plurality of papers.
Another reason why the multi-pass dot recording method for a serial printer is not applicable to a line printer with a rotary drum is that if the number of passes is increased to improve the clearness of the image in high-speed printing such as double or quadruple speed with a serial printer, the number of times of scanning of the nozzle head would increase, so that the throughput (printing time per sheet) increases considerably. On the other hand, because a line printer with a rotary drum is designed for high-speed mass-printing, such a system requiring long throughput is not applicable to high-speed printing.
If the scanning speed of the carriage of the nozzle head was increased to prevent the throughput from decreasing, the carriage would have to move at ultra-high speed and the acceleration and the deceleration at both ends of its travel would become extremely large. This would require a mechanical structure that can withstand large acceleration and deceleration. This would make the apparatus bulky and the manufacturing cost would increase so as to increase the strength and accuracy. The durability would decrease if the strength and accuracy are not increased. Also, because the areas for acceleration and deceleration are needed for the travel of the carriage, stroke ranges which do not contribute the formation of images have to be provided at both ends of the stroke which is needed for the formation of images. If the scanning speed is increased to keep low the deceleration, the strokes at both ends would be remarkably large.
Such an increase of the acceleration and deceleration at both ends of carriage travel can cause trouble in a smooth supply of ink from the ink tank in the nozzle head to the nozzles. Also, moving the carriage at high speed and accelerating and decelerating it increases the vibration and noise of the apparatus. One approach is to keep the nozzle heads standstill or move them at such a low speed as not to cause such problems and perform multi-pass dot recording in the sub-scanning direction at high speed such as double or quadruple speed on a plurality of papers mounted on a rotary drum. But such an approach has not been proposed.
On the other hand, returning to the deterioration of quality, the first cause is that the volume (amount) of ink jetted from the nozzles and the ink jet direction are not uniform but uneven.
With an inkjet line printer provided with a line head having many nozzles arranged in lines, particularly a line head having short heads mounted in staggered fashion (e.g. carriage 10 used in FIGS. 3, 15 and 28 in the embodiments), stripe-like unevenness can appear at portions between the short heads due to a difference in the scanning direction (perpendicular to the drum rotating direction) of the short heads.
As one solution of this problem, Patent publication 4 discloses a color inkjet printer for color printing by a line head on a paper mounted on a rotary drum. This printer has a line head arranged opposite to the rotating surface of a drum on which a printing medium is mounted, the line head having short heads arranged in staggered fashion so as to be perpendicular to the drum rotating direction. The printer completes one image by printing (image recording) a plurality of times.
Namely, printing (recording) is done every n−1 (n>2) pixel in at least one of the main scanning direction and sub-scanning direction, and in one printing (one rotation) in the main scanning direction ink is jetted from every n−1 (n>2) nozzle. In printing, recording is done so as for dots not to overlap, thereby suppressing unevenness resulting from the mixing of ink or wetting of printing paper. Further, by moving the line head in the main scanning direction, unevenness in printing due to misfired nozzles and a difference between dots with nozzles is dispersed, thereby improving the image quality by multi-pass system.
With such an inkjet printer, if the moving distance in moving the line head in the main scanning direction can be increased, it is possible to increase the effect of multi-pass in improving the quality and achieve high image quality printing. This is because if unevenness is formed due to non-printing by misfired nozzles, small moving distance of the line head results in insufficient dispersion of unevenness, thus making unevenness noticeable to the human eye.
However, the larger the moving distance, the longer time is taken for moving the head. Also, it is required that movement of the head be started after printing (image recording) is complete and it ends before the next printing starts. Therefore, the speed of the drum is regulated by the moving speed of the head. So the drum speed had to be decreased. This makes high speed printing impossible.
Increasing the moving speed of the line head is considered to be one solution. But, increasing the head speed would apply undue force to the ink in the head (like the above serial printer), thus impairing the inkjet performance by the effect of change in pressure in the ink tank and making high-image quality printing impossible.
Other causes of deterioration of image quality are as follows.
Inkjet printers, one of the line dot recorders, jet fine ink particles from inkjet nozzles of a micron size to paper for printing. Therefore, inkjet printers are liable to cause deterioration of print image due to clogging of inkjet nozzles with ink or dust. Particularly, with line printers which have inkjet nozzles arranged in lines to print one line at one time, the number of inkjet printers requiring cleaning is large and the line heads are of a large size in comparison with those of serial printers. Various cleaning mechanisms have been proposed to solve these troubles.
For example, with the line printer disclosed in Patent publication 5, as shown in FIG. 33, the line head 1 has one end thereof supported by a pivoting shaft so that it can be turned by 90 degrees from its recording position to home position H for cleaning. After being moved, the line head 1 is subjected to cleaning by means of a cleaning unit U standing by the home position H. The cleaning unit U has a cleaning blade and a suction cap. Firstly the cleaning blade wipes the line head 1 to remove ink and dust clinging to the surface of the line head and then the suction cap is pressed against the line head to suck ink and dust out of the ink nozzles.
However, with this inkjet printer, nozzles for all colors are provided on a single line head. The line head that has to be turned is single, but, in order to perform printing with high accuracy at high speed, the number of nozzles has to be increased. Therefore, if one line head is provided for each color, at least four (eight if the line heads are divided into two groups like embodiment 2 in Patent publication 5) line heads 1 have to be turned. This can cause the line heads to butt each other in turning. Also, a complicated mechanism becomes necessary to avoid collision of the line heads. Another problem is that a large space is required to turn a plurality of line heads.
Further, with this inkjet printer, ink is supplied from an ink cartridge mounted on the line head 1. If an external tank is used, the piping from the external tank to the line head 1 will be needed by the same number as the number of the line heads. Since the line heads have to be turned, the piping to the line heads will be difficult to arrange.
[Patent publication 1] Japanese patent publication 2001-18374
[Patent publication 2] Japanese patent publication 11-115220
[Patent publication 3] Japanese patent publication 4-366645
[Patent publication 4] Japanese patent publication 2002-11865
[Patent publication 5] Japanese patent publication 2002-103638